In analog circuits fabricated using complementary metal oxide semiconductor (CMOS) techniques, it is common to require a comparator as an output circuit. Such a circuit receives a small signal analog or alternating current (AC) input signal and produces a large digital signal as an output signal. The input signal is compared with a threshold value to produce the output signal. The comparator may be used as an output circuit for interfacing to another integrated circuit, or may provide level conversion. An example of such a circuit is found in automatic gain control (AGC) circuits where detected output power is compared to a known direct current (DC) level and a digital signal is produced to turn on or off an external step attenuator.
Such comparator circuits have a number of operational and performance requirements. The comparator must have hysteresis so that the output signal level will not oscillate between high and low levels due to circuit noise or minute input signal level change. Further, the comparator circuit must have high gain to provide fast output signal transitions. Lastly, the circuit should draw very little current and occupy very little surface area of an integrated circuit.
Accordingly, there is a need in the art for a low power CMOS comparator circuit with high gain and hysteresis.